The present invention relates generally to image generating devices, and more particularly, to compact display systems, such as those used in head mounted displays.
Head mounted displays have received considerable attention as a technique for displaying high magnification, large field of view, and high definition virtual images. The head mounted display generally includes a support member for mounting the display on a head of a user and various optical and display components. The components are arranged to magnify an image displayed on a compact image display panel (microdisplay) such as a liquid crystal display (LCD) and to display the magnified image ahead of the user through the optical system. The user typically does not directly observe an image displayed on a monitor or screen, but instead observes a magnified virtual image converted from the image displayed on the display panel. The head mounted display thus provides a compact arrangement for displaying to the user a larger image than displayed on a small microdisplay panel.
Recently, microdisplay devices have been developed that produce a viewable image generally no greater than 4.8 mm.times.3.6 mm in size. This is advantageous in terms of reducing manufacturing cost, however, problems have been encountered in attempting to magnify these small images for comfortable viewing by a user. Typically, such displays have an emission angle of approximately 10 to 15 degrees. For example, if the microdisplay device is based on liquid crystal technology, there is only a narrow angle over which a high contrast image is viewable. An optical system used to magnify the image displayed on the microdisplay must provide a sufficiently large exit pupil. The exit pupil needs to be large enough to allow for rotations and side to side movement of the eye and slippage of the display. Exit pupil size usually has to be traded off against field of view. However, due to the Lagranage invariant of classical optical theory, the size of the exit pupil and field of view must be constrained as follows: EQU Exit Pupil Dimension.times.Field of View=Display Dimension.times.Emission Angle.
Since both the display dimension and the emission angle are small, the viewing geometry is typically limited to an exit pupil of 6 mm diameter and a field of view of 40.degree.. If larger fields of view are required, the exit pupil may be too small to be usable for comfortable viewing of the image.
Furthermore, it is often difficult to provide a sufficiently bright image with a microdisplay. In order to produce a multicolor image with a microdisplay that produces only monochrome images, a sequence of images are typically displayed and illuminated sequentially with red, green, and blue lights produced by a light source (e.g., array of light emitting diodes (LEDs)). The projected emission area of the light source on the surface of the microdisplay must generally conform to the size of the microdisplay. This typically results in a displayed image that is insufficiently bright.
There is, therefore, a need for an image generating device for use with a microdisplay, which provides a relatively large field of view and exit pupil and a sufficiently bright image that can be comfortably viewed by a user with a compact wearable display.